1. Field of the Invention
The present invention relates to a four-pole monolithic filter having two monolithic filter elements formed on a single piece of a piezoelectric blank.
2. Description of the Related Art
A monolithic filter is provided with a plurality of pairs of electrodes, which are formed in close proximity to one another on a single piezoelectric blank or plate in such a manner that a vibration mode of one vibrator is coupled to a vibration mode of the other vibrator so as to transmit a vibrating energy from one vibrator to the other vibrator thereby creating an acoustic coupling and transmitting only a vibration energy of desired frequency component. A crystal blank or crystal plate may, for example, be used as the piezoelectric blank. This type of monolithic filter has been widely used in recent years.
When the monolithic filter is requested to exhibit better attenuation or damping characteristic, as shown in FIG. 1, two monolithic filters 11 and 12 are connected in cascade, and a capacitor 13 halving a certain load capacitance is connected between the point of cascade connection of the two filters and an electric ground potential. For example, Japanese Laid-open Patent Publication (Kokai) No. Hei-9-46170 (JP, 09046170, A) discloses two monolithic filters, which are connected in cascade and encased in a single encasing container.
This JP, 09046170, A discloses a first example of two monolithic filters in cascade connection, in which two monolithic filters are separately formed on different piezoelectric blanks, and each of the piezoelectric blanks is fixed, at three corners thereof, to the encasing container by means of conductive adhesive. Further, the two filters are electrically connected in cascade to one another, and are also electrically connected to connecting terminals provide for the encasing container.
Nevertheless, in a monolithic filter employing a crystal blank as the piezoelectric blank and having a central frequency of approximately 80 MHz, especially in the small type monolithic filter, the size of the piezoelectric blank is of approximately 3 mm square. Therefore, there occurs such a problem that an accurate positioning of the two of such small piece of crystal blanks in place in the container and fixing thereof to the container are quite cumbersome in assembling while requiring a high production cost.
The above-mentioned JP, 09046170, A also discloses, as a second example having two monolithic filters in cascade connection. In the example, two couples of electrodes are formed on a single piece of piezoelectric blank so as to form two monolithic filter elements connected in cascade on the piezoelectric blank. In the disclosed filter, the piezoelectric blank is formed by a rectangular plate, and the piezoelectric blank is fixed at six positions, i.e., at four respective corners of the blank and at two respective central positions of the long sides of the rectangle, to the encasing container by means of conductive adhesive.
Nevertheless, in the case of such constructed filter, in order to adjust the attenuating characteristic of respective monolithic filter elements and also in order to connect a load capacitance to the connecting point of the series connection of the two filter elements, input and output electrodes must be led out separately for every one of the two filter elements. Accordingly, the number of terminals to be led out is five in total, including at least four at input and output electrodes and one at a grounded electrode. Further, if the ground electrode is led out from every one of the filter elements from the view point of making it easy to obtain a balanced condition in high frequency region, six electrodes in total must be led out of the two filter elements.
On the other hand, when the electrodes are led out of the blank, it is desired that a distance between respective two of the electrodes is as large as possible in order for avoiding unnecessary coupling of high frequency signals. Hereunder, any mutual couplings among the electrodes at a high frequency region can be reduced to the least possible extent to result in acquiring a good attenuating characteristic. Therefore, when six electrodes are led out, these electrodes are led out of six positions, i.e., the respective four corners of the crystal blank and the respective central positions of the two long sides of the blank.
The present inventor, as shown in FIG. 2, practically manufactured an example having two monolithic filter elements formed on a single piece of crystal blank, and conducted measurement of the characteristic of the example.
The manufactured filter is constructed in such a manner that on a rectangular crystal blank of 5 mmxc3x972.5 mm extent, two monolithic filter elements 15 and 16 having a central frequency of approximately 80 MHz are formed, and that the two monolithic filter elements are connected in cascade so as to form a surface mount type four-pole monolithic filter. As will be understood from the illustration of FIG. 2, this filter is constructed so that crystal blank 14 is encased in box-like container 17. Crystal blank 14 is provided with six positions where electrodes are led out, and these electrodes are fixed to six holding electrodes 18 formed in six positions of the bottom face inside container 17 by means of conductive adhesive 19 so as to hold crystal blank 14 within container 17. A plurality of four-pole monolithic filters, each having the above-described construction, were manufactured and with the attenuating characteristic of each of the manufactured filters, measurement of a temperature characteristic at the central frequency was conducted. The result of the measurement is shown in FIG. 3.
As shown in FIG. 3, according to the prior art four-pole monolithic filter in which the crystal blank is held at its six positions by means of conductive adhesive, a change in the central frequency in the attenuating characteristic against a temperature change is very large with reference to the central frequency at the temperature of +25xc2x0 C., when a change in the central frequency in the temperature range from xe2x88x9230xc2x0 C. through +80xc2x0 C. was measured, a change of approximately 35 to 80 ppm occurs at xe2x88x9230xc2x0 C. particularly on a low temperature side, and a change on average was 50 ppm. This characteristic of the prior art four-pole monolithic filter obviously cannot satisfy the standard that is required for, for example, recent portable radio appliances, e.g., a portable telephone, with respect to the temperature change.
An object of the present invention is to provide a four-pole monolithic filter, which is small in its change in the central frequency in the attenuating characteristic against a temperature change and is able to acquire a stable characteristic.
The present inventor has conducted various studies to investigate a cause for the change in the central frequency of the above-described type four-pole monolithic filter. As a result, due to a difference in the factor of thermal expansion between the container made of ceramic and the crystal blank, a stress acts on the crystal blank via the conductive adhesive for the fixing of the crystal blank due to the temperature change, so that a phenomenon making it possible to assume that the change in the central frequency occurs by the affect of the action of the stress was founded. Thus, the present inventor has detected that a change in the central frequency versus a temperature change can be mitigated by reducing the number of supporting points of the piezoelectric blank such as a crystal blank, and accomplished the present invention.
In other words, the object of the present invention is attained by a four-pole monolithic filter having a constitution in which two sets of monolithic filter elements are formed on a single piezoelectric blank, the monolithic filter elements being electrically connected in cascade to one another, and comprising: a rectangular piezoelectric blank; a container for encasing therein the piezoelectric blank; first and second connecting electrodes respectively formed in first and second corners positioned on an identical diagonal line of the piezoelectric blank; first and second monolithic filter elements arranged to be mutually electrically connected in cascade between the first and second connecting electrodes on the piezoelectric blank; a third connecting electrode arranged at a central portion of a first long side of the piezoelectric blank for permitting a connecting point for the cascade connection of the first and second monolithic filter elements to be led out thereto; a fourth connecting electrode arranged at a central portion of a second long side of the piezoelectric blank for permitting ground electrodes of the first and second monolithic filter elements to be led out thereto; and first to fourth holding electrodes formed in the container to correspond to the first to fourth connecting electrodes, wherein the first to fourth connecting electrodes are respectively fixed to the first to fourth holding electrodes by means of conductive adhesive thereby holding the piezoelectric blank in the container and establishing electrical conduction of the respective first to fourth connecting electrodes to the first to fourth holding electrodes.